1. Field of the Invention
The present invention relates to control systems and, more particularly, to control of rectifiers employing semiconductor devices, such as silicon controlled rectifiers (SCRs) or thyristors. The invention also relates to a method for controlling a rectifier bridge.
2. Background Information
Three-phase rectifier circuits are commonly employed to convert AC signals to DC signals. These circuits often use SCRs disposed in bridge segments, with typically one SCR for each polarity of each AC phase. Typically, a bridge firing control circuit controls the firing point for each rectifier in each AC cycle. Examples of such circuits are disclosed in U.S. Pat. Nos. 5,963,440; 5,963,441; 6,046,917; 6,208,120; and 6,232,751.
It is not uncommon for a plurality of SCR bridges to be operated in parallel with each of the corresponding bridge firing control circuits being controlled by a central firing control circuit. The central firing control circuit manages each of the bridge firing control circuits in order that the corresponding rectifiers in each of the parallel bridges conduct current at the same point in the AC waveform.
SCR bridges are commonly employed in an excitation control system to provide field excitation for a rotating electrical apparatus (e.g., large synchronous generators and motors, utility synchronous generators and motors, industrial synchronous motors and generators, synchronous generators and motors for naval or other shipping applications, synchronous generators and motors for oil well drilling rigs).
Traditionally, fuses are employed in series with each one of the semiconductor devices (e.g., six total thyristors or SCRs) in a full-wave (e.g., six-pulse) bridge, in order to protect the bridge and the semiconductor devices from faults. A bridge converter may include two or more parallel bridges. Each one of these bridges is configured in parallel with the other one or more bridges, in order that they share the load current. Furthermore, one or two fuses may be employed between the DC output of each one of the bridges and the load.
Using only AC line fuses (e.g., three total for a three-phase power source) on converter bridges reduces the number of fuses required and generally protects both the AC bus and the DC bus. However, other problems may result.
When the load is inductive, the instantaneous output of the bridge (i.e., the load voltage) may be negative during certain intervals (although the load current is generally constant through the inductive load). If AC line fuses are employed, and if a semiconductor device (e.g., thyristor) is shorted (i.e., failed), then a path for current is provided through the shorted thyristor when the conjugate (i.e., series) thyristor is gated. This is because the negative voltage may be present across the load and, hence, current would then flow through the shorted thyristor when the conjugate thrysitor is fired. This is unacceptable because continued operation may result in the conjugate thyristor failing if it carries too much current. Also, the converter bridge cannot develop full negative voltage.
Accordingly, there is room for improvement in controlled rectifier bridges, control systems, and methods for controlling rectifier bridges.